The present invention relates in general to pulverized fuel delivery systems such as pulverized coal injection (PCI) systems for blast furnaces used in iron and steel production, and in particular to a new and unique pulverized fuel delivery system and method which uses a high pressure, variable speed solids pump for continuously providing pulverized coal to one or more blast furnaces or other users of pulverized coal.
The use of pulverized coal as a fuel for blast furnaces was first introduced approximately 35 years ago, and is a popular fuel due to its relatively low cost and widespread availability. Several different delivery systems for conveying the pulverized coal to furnaces or other combustion applications have been developed. In particular, one modern group of substantially continuous flow, high pressure pulverized coal pneumatic delivery systems is characterized by the use of atmospheric reservoirs to fill pressurized feeder tanks, which in turn supply pulverized coal to multiple injection lines or to a feed line connected to one or more distributors. The distributors convey the pulverized coal from the feed line to multiple points in a furnace or other application. The coal may be provided in what is known as "dense phase" because of the relatively high ratio of solids to volume of gas present, or it may be conveyed in dilute phase depending on the specific technology employed.
However, these known methods for continuously delivering pulverized coal fuel to blast furnaces for burning all have drawbacks, such as inefficient use of materials, space, or energy. These problems arise primarily from the difficulty of moving the pulverized coal from atmospheric pressure storage bins to higher pressure feeder or batch tanks for injection into a furnace. Also, because the pulverized coal is provided in the dense phase at high pressure, rotary feeders do not work well due to pressure limitations.
U.S. Pat. Nos. 3,689,045 and 3,720,351 to Coulter et al. both disclose a pulverized coal delivery system for providing dense phase pulverized coal to a blast furnace. An atmospheric coal grinding and collection system is combined with two or more pressurized batch or feeder tanks, preferably at least three separate feeder tanks, which are connected to one storage reservoir. While one full feeder tank is used to supply the pulverized coal to the blast furnace at high pressure, the remaining two feeder tanks may be refilled from the storage reservoir at atmospheric pressure. Once a feeder tank is filled, it is pressurized and readied to be placed online when the supply of pulverized coal in that feeder tank currently feeding the blast furnace is depleted, thus maintaining a substantially continuous pulverized coal fuel flow into the blast furnace. This cycle is continuously repeated, such that one feeder tank is always online and feeding the blast furnace, while the remaining two feeder tanks are at varying stages of refilling with pulverized coal and/or recharging to high pressure.
More particularly, the pulverized fuel delivery systems of Coulter et al. operate such that each batch tank in these systems is cycled continuously in the following sequence:
a. At atmospheric pressure (vented), the feeder tank is filled by gravity flow from a pulverized fuel reservoir located above through a connecting pipeline. PA1 b. Once filled, a valve in the fill pipeline is closed and the feeder tank is pressurized with inert gas. PA1 c. Once pressurized, the feeder tank is in the ready condition and remains in standby until the on-line feeder tank is empty. PA1 d. When the time comes for the ready tank to go on-line, e.g., to begin feeding pulverized fuel to the blast furnace, a valve in the discharge line located below the tank opens and pulverized fuel in dense phase flows out under pressure into the fuel transport and distribution system which connects the tank to the furnace. PA1 e. Once the tank is nearly empty, the pulverized fuel discharge valve closes and the feeder tank pressure is vented down to atmospheric pressure. This completes the cycle which generally requires a time span of 30 to 90 minutes.
Another common form of high pressure solids feed system employs two tanks in series, and is shown in schematically in FIG. 1. The first tank, commonly referred to as a lock hopper, receives solids materials from an atmospheric storage reservoir by gravity flow. This first tank is then closed and pressurized to a pressure equal to the pressure of the second or feed tank. A drain valve in the first tank is opened to release the material into the feed tank. Once the first tank is emptied, it is depressurized and refilled for another cycle.
Other known methods for continuously transporting fine solids in dense phase include the cascading pressure continuous blow bottle disclosed by U.S. Pat. No. 5,265,983 to Wennerstrom et al. The Wennerstrom et al. patent provides for the continuous filling of a blow bottle, which takes the place of multiple feeder tanks. This device employs a single variable speed rotary feeder in combination with one or more constant speed rotary feeders in a cascade arrangement. The upper variable speed rotary feeder is capable of handling 20 psig differential pressure, while the lower constant speed feeders are designed for higher differential pressures up to 50 psig. Continuous venting of the rotary feeders is necessary to prevent up-draft of gas through the feed system. In a high pressure system, the continuous venting of the feeders will result in a large quantity of compressed gas (typically nitrogen or N.sub.2) being lost, and this wasted nitrogen is a costly element in the overall system.
U.S. Pat. No. 4,392,438 to Dooley discloses a coal transport system for delivering a pulverized coal fuel from a remote point directly to a furnace or alternately to a storage chamber. The system disclosed in the '438 patent uses coal gas to pressurize the system and force the pressurized coal from a processing and pulverizing plant through a pipeline having a series of booster stations used to maintain pressure to a furnace. The system of the '438 Dooley patent is similar in concept to that of the present invention, however it does not use a high pressure variable speed solids pump to maintain and initiate the fuel flow into the furnace, nor does it concern itself with filling and maintaining a fuel level in a feeder tank.
U.S. Pat. No. 5,285,735 to Motoi et al. discloses a control apparatus for injection of a particular quantity of pulverized coal into a blast furnace. This patent does not disclose the use of a high pressure variable speed solids pump either, but merely a different means of controlling the level of coal in a feed tank for supplying the furnace. The Motoi et al. patent uses additional pressurizing gas to maintain the pressure within the feed tank while varying the rate at which the feed tank is filed with the control system. The Motoi et al. patent's apparatus uses a conveying gas in conjunction with a pressurized gas and a series of valves to achieve similar results as are achieved with the high pressure pump of the present invention which requires much less equipment.
It is thus apparent that an improved pulverized fuel delivery system that can reduce or eliminate: the cycling of multiple batch tanks, the disruptions that occur when one such batch tank is taken off line and another is started, and the venting of significant quantities of pressurizing gas, would be welcomed by the industry.